In operating an internal combustion engine, combustion data relating to normal combustion characteristics such as the timing for start of combustion (“SOC”) and the combustion rate, which can be derived from the heat release rate and the in-cylinder pressure, are combustion characteristics that can be used to improve the combustion quality, increase engine efficiency and reduce undesirable combustion products, such as soot (also known as particulate matter), unburned fuel, NOx and greenhouse gases. More accurate combustion data relating to at least one of these combustion characteristics is desirable to improve engine operation. Known instrumentation for measuring combustion data include: pressure transducers that employ sensors disposed in the combustion chamber to measure in-cylinder pressure; emissions sensors that can be located in the exhaust ports or exhaust manifold; and temperature sensors located in the combustion chamber or in an exhaust port. Such instrumentation can be used to collect data that can be correlated with combustion characteristics or combustion data directly. However, it can be impractical to employ such instrumentation in a mass produced engine, because in addition to being intrusive and/or being subjected to a harsh environment in the location where they are installed, such sensors can also be expensive and lack the durability needed for reliable and prolonged everyday use.
Vibration sensors, such as for example accelerometers, are sold commercially for use as knock sensors for detecting engine knock. Engine knock is considered an abnormal combustion characteristic because it does not occur when the engine is operating normally. Engine knock correlates to violent combustion events that can be caused by pre-mature and uncontrolled detonation of the charge inside the combustion chamber, caused, for example, if an Otto-Cycle engine has been fuelled with a fuel that has an octane rating, which is too low, or if spark timing is too early, or if deposits in a combustion chamber create hot spots that cause early ignition. In a Diesel-Cycle engine, engine knock can be caused, for example, if fuel injection timing is too early. Engine knock can cause serious damage to the engine. Vibration sensors that are employed as “knock sensors” are typically located on an engine's cylinder block and sometimes on the cylinder head. As an abnormal combustion characteristic, engine knock is not a combustion characteristic that is measured in each engine cycle. Most knock sensors send a base or “no knocking” reference signal to the electronic engine controller and an easily detectable higher signal when engine knock is detected. Because there is a relatively large difference between the reference signal and the signal when engine knock is detected the accuracy of the knock sensor is relatively unimportant. In this respect, knock sensors are only required to make crude measurements since they are configured to detect only engine knock.
More recently, it has been found that by using techniques, for example, to increase the signal-to-noise ratio and to filter out signal noise, it is possible to use vibration sensors to extract a signal that can be used to determine a normal combustion characteristic. The vibration sensor is not exposed to the harsh conditions inside the combustion chamber, but because it is mounted to a component of the engine, and there are many moving parts in an internal combustion engine and in the machinery that the engine is associated with, such as a moving vehicle, the vibrations that the vibration sensor detects include vibrations caused by sources other than combustion. Without processing the raw signal from a vibration sensor to filter out signal noise, the raw signal can not be used as a sufficiently accurate indicator of combustion characteristics other than engine knock. An accurate determination of a combustion characteristic is required in order to improve engine performance and efficiency by adjusting parameters such as, for example, the timing for fuel injection, the fuel injection rate, and the quantity of fuel injected.
A vibration signal can be conditioned and digitized, for example as set out in co-owned U.S. Pat. Nos. 7,133,761 and 7,200,487, respectively entitled “Method and Apparatus for Controlling an Internal Combustion Engine Using Accelerometers”, and, “System and Method for Processing an Accelerometer Signal to Assist in Combustion Quality Control in an Internal Combustion Engine”. These patents teach that it is possible to use the signal from an accelerometer sensor to directly detect different operating parameters of a diesel engine (for example, the timing for start of combustion (SOC)).
While it can be important to know the timing for SOC to help control combustion in an internal combustion engine, the quality of combustion, combustion efficiency, and engine performance can be better controlled and improved if more combustion characteristics are accurately known for a broad range of the engine cycle, such as the heat release rate profile or the in-cylinder pressure profile during combustion for each cylinder.
Some of the challenges associated with methods for reconstructing the heat release rate profile or the in-cylinder pressure profile for one or more engine cycles for an engine include: engine-to-engine variability, cylinder-to-cylinder variability, and sensor-to-sensor variability. Another challenge associated with such methods is compensating for changes in sensor sensitivity between different engine speeds or load conditions, and changes in sensor sensitivity over time. The sensor performance and characteristics (for example, impedance, sensitivity) can vary from one vibration sensor to another. Also, errors can be introduced if a sensor's charge decay is not taken into account. None of the known references describes a method of reconstructing in-cylinder pressure that includes correcting the vibration signal for the sensor's sensitivity and charge decay. For sensors generally, a known method of addressing charge decay comprises calibrating the sensor, whereby prior knowledge about the sensor's characteristics, including charge decay, is accounted for. Another method for addressing the issue of charge decay in sensors is using a charge amplifier to avoid operating the sensor in voltage mode. None of these methods can compensate for a sensor's charge decay or changes in sensitivity over a dynamic range of different engine conditions to improve processing of sensor signals for better accuracy of the reconstructed combustion characteristics such as heat release rate data or in-cylinder pressure data during combustion for each cylinder.
Accordingly, the utility of methods that use vibration sensors for controlling combustion can be improved if the method for processing of the vibration sensor signal can produce more accurate data pertaining to combustion characteristics such as heat release rate and in-cylinder pressure over a broader range of the combustion portion of the engine cycle for each cylinder. In particular, the processing method can be improved if it can eliminate errors introduced by charge decay and the sensitivity of the vibration sensor.